1. Field of the Invention
This present invention relates to a power conversion circuit for driving fluorescent lamps, and, more particularly, relates to circuitry in the power conversion circuit which controls the minimum brightness of the fluorescent lamps.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFLs are used for back or edge lighting of liquid crystal displays (LCDs) which are typically used in notebook computers, web browsers, automotive and industrial instrumentation, and entertainment systems.
CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. After an initial ignition stage and the formation of plasma, current flows through the tube which results in the generation of ultraviolet light. The ultraviolet light in turn strikes a phosphoric material coated in the inner wall of the tube, resulting in visible light.
A power conversion circuit is used for driving the CCFL. The power conversion circuit accepts a direct current (DC) supply voltage and provides a substantially sinusoidal output voltage to the CCFL. The brightness of the CCFL is controlled by controlling the current (i.e., lamp current) through the CCFL. The lamp current can be amplitude modulated or time modulated for dimming control of the CCFL. Time modulation typically offers a wider dimming range.
The lamp current is time modulated by selectively turning off the sinusoidal output voltage provided to the CCFL for varying time durations. For example, the sinusoidal output voltage alternates between being on for Tx seconds and being off for Ty seconds. The period (i.e., summation of Tx and Ty) is generally fixed in constant frequency operation to reduce electro-magnetic-field (EMF) interference with other devices. The on-time duty cycle (i.e., Tx/(Tx+Ty)) determines the brightness of the CCFL. Maximum brightness results when the sinusoidal output voltage is on all the time with a 100% duty cycle (i.e., Ty=0). Minimum brightness results when the duty cycle is small (i.e., Ty>>Tx).
A wide dimming range is desirable for efficient operation of the CCFL. The dimming range of the CCFL is generally limited by the minimum brightness that can be achieved without flickering or shimmering. To achieve minimum brightness without flickering or shimmering, the on-time of the sinusoidal output voltage needs to be the minimum time possible to produce a lamp current with a minimum number of cycles with respective amplitudes above a preset threshold.
Each lamp current cycle corresponds to a respective cycle of the sinusoidal output voltage. Ideally, each cycle of the sinusoidal output voltage produces a lamp current cycle with a respective amplitude above the threshold. However, lamp characteristics, LCD mechanical structure, operating temperature and supply voltage variations can cause the amplitudes of some of the initial lamp current cycles to fall below the threshold, thereby causing flickering or shimmering.
Prior art systems set the minimum on-time of the sinusoidal output voltage to a sufficiently long time such that the number of lamp current cycles with respective amplitudes above the threshold is equal to or greater than the required minimum number under all operating conditions. Under most conditions, the CCFL is operating above the minimum brightness with the minimum on-time setting to avoid undesired flickering or shimmering. The dimming range of the CCFL is effectively limited.